1892.] 071 Alternate Currents of High Potential and Frequency. 643 



dielectric, even several times thicker tlian tlie oil intervening 

 between the metal plates, is inserted between the latter, the air having 

 free access to the charged surfaces, the dielectric invariably is warmed 

 and breaks down. 



The employment of the oil is advisable or necessary even with 

 low frequencies, if the potentials are such that streamers form, but 

 only in such cases, as is evident from the theory of the action. If 

 the potentials are so low that streamers do not form, then ft is even 

 disadvantageous to employ oil, for it may, principally by confining 

 the heat, be the cause of the breaking down of the insulation. 



The exclusion of gaseous matter is not only desirable on account 

 of the safety of the apparatus, but also on account of economy, 

 especially in a condenser, in which considerable waste of power may 

 occur merely owing to the presence of air, if the electric density on 

 the charged surfaces is great. 



In the course of these investigations a phenomenon of special 

 scientific interest has been observed. It may be ranked among the 

 brush phenomena, in fact it is a kind of brush which forms at, or 

 near, a single terminal in high vacuum. In a bulb with a conducting 

 electrode, even if the latter be of aluminium, the brush has only a 

 very short existence, but it can be preserved for a considerable length 

 of time in a bulb devoid of any conducting electrode. To observe 

 the phenomenon it is found best to employ a large spherical bulb 

 having in its centre a small bulb supported on a tube sealed to the neck 

 of the former. The large bulb being exhausted to a high degree, and 

 the inside of the small bulb being connected to one of the terminals of 

 the coil, under certain conditions there appears a. misty haze around 

 the small bulb, which, after passing through some stages, assumes the 

 form of a brush, generally at right angles to the tube supporting the 

 small bulb. When the brush assumes this form it may be brou<^ht to 

 a state of extreme sensitiveness to electrostatic and magnetic influence. 

 The bulb hanging straight down, and all objects being remote from 

 it, the approach of the observer within a few paces will cause the 

 brush to fly to the opposite side, and if he walks around the bulb it 

 will always keep on the opposite side. It may begin to spin around 

 the terminal long before it reaches that sensitive stage. When it 

 begins to turn around, principally, but also before, it is affected by a 

 magnet, and at a certain stage it is susceptible to magnetic influence 

 to an astonishing degree. A small permanent magnet, with its poles 

 at a distance of no more than two centimetres, will affect it visibly 

 at a distance of two metres, slowing down or accelerating the rotation 

 according to how it is held relatively to the brush. 



When the bulb hangs with the globe down, the rotation is always 

 clockwise. In the southern hemisphere it would occur in the opposite 

 direction and on the (magnetic) equator the brush should not turn at 

 all. The rotation may be reversed by a magnet kept at some distance. 

 The brush rotates best, seemingly, when it is at right angles to the 

 lines of force of the earth. It very likely rotates, when at its maximum 



